Soldering Basics
SOLDERING PROCESS
Soldering is the process of joining two metals by the use
of a solder alloy, and it is one of the oldest known joining
techniques. Faulty solder joints remain one of the major
causes of equipment failure and thus the importance of
high standards of workmanship in soldering cannot be
overemphasized.
The following material covers basic soldering procedures
and has been designed to provide the fundamental
knowledge needed to complete the majority of high
reliability hand soldering and component removal
operations.
Figure 1: Wetting occurs when
molten solder penetrates a
copper surface, forming an
intermetallic bond.
PROPERTIES OF SOLDER
Solder used for electronics is a metal alloy, made by
combining tin and lead in different proportions. You can
usually find these proportions marked on the various
types of solder available.
With most tin/lead solder combinations, melting does not
take place all at once. Fifty-fifty solder begins to melt at
183°C (361°F), but it's not fully melted until the
temperature reaches 216°C (420°F). Between these two
temperatures, the solder exists in a plastic or semi-liquid
state.
Figure 2: Minimal thermal linkage
due to insufficient solder between
the pad and soldering iron tip.
The plastic range of a solder varies, depending upon the
ratio of tin to lead. With 60/40 solder, the range is much
smaller than it is for 50/50 solder. The 63/37 ratio, known
as eutectic solder has practically no plastic range, and
melts almost instantly at 183°C (361°F).
The solders most commonly used for hand soldering in
electronics are the 60/40 type and the 63/37 type. Due to
the plastic range of the 60/40 type, you need to be careful
not to move any elements of the joint during the cool down
period. Movement may cause what is known as disturbed
joint. A disturbed joint has a rough, irregular appearance
and looks dull instead of bright and shiny. A disturbed
solder joint may be unreliable and may require rework.
Figure 3: A solder bridge
provides thermal linkage to
transfer heat into the pad and
component lead.
WETTING ACTION
When the hot solder comes in contact with a copper surface, a metal solvent action takes
place. The solder dissolves and penetrates the copper surface. The molecules of solder and
copper blend to form a new alloy, one that's part copper and part solder. This solvent action is
called wetting and forms the intermetallic bond between the parts. (See Fig. 1). Wetting can
only occur if the surface of the copper is free of contamination and from the oxide film that
forms when the metal is exposed to air. Also, the solder and work surface need to have
reached the proper temperature.
Although the surfaces to be soldered may look clean, there is always a thin film of oxide
covering it. For a good solder bond, surface oxides must be removed during the soldering
process using flux.
FLUX
Reliable solder connections can only be accomplished
with truly cleaned surfaces. Solvents can be used to
clean the surfaces prior to soldering but are insufficient
due to the extremely rapid rate at which oxides form on
the surface of heated metals. To overcome this oxide
film, it becomes necessary in electronic soldering to use
materials called fluxes. Fluxes consist of natural or
synthetic rosins and sometimes chemical additives called
activators.
Figure 4: Solder blends to the
soldered surface, forming a small
contact angle.
It is the function of the flux to remove oxides and keep
them removed during the soldering operation. This is
accomplished by the flux action which is very corrosive at solder melt temperatures and
accounts for flux's ability to rapidly remove metal oxides. In its unheated state, however, rosin
flux is non-corrosive and non-conductive and thus will not affect the circuitry. It is the fluxing
action of removing oxides and carrying them away, as well as preventing the reformation of
new oxides that allows the solder to form the desired intermetallic bond.
Flux must melt at a temperature lower than solder so that it can do its job prior to the
soldering action. It will volatilize very rapidly; thus it is mandatory that flux be melted to flow
onto the work surface and not be simply volatilized by the hot iron tip to provide the full benefit
of the fluxing action. There are varieties of fluxes available for many purposes and
applications. The most common types include: Rosin - No Clean, Rosin - Mildly Activated and
Water Soluble.
When used, liquid flux should be applied in a thin, even coat to those surfaces being joined
and prior to the application of heat. Cored wire solder and solder paste should be placed in
such a position that the flux can flow and cover the joints as the solder melts. Flux should be
applied so that no damage will occur to the surrounding parts and materials.
SOLDERING IRONS
Soldering irons come in a variety of sizes and shapes. A continuously tinned surface must be
maintained on the soldering iron tip's working surface to ensure proper heat transfer and to
avoid transfer of impurities to the solder connection.
Before using the soldering iron the tip should be cleaned by wiping it on a wet sponge. When
not in use the iron should be kept in a holder, with its tip clean and coated with a small
amount of solder
NOTE
Although tip temperature is not the key element in soldering you should always start at the
lowest temperature possible. A good rule of thumb is to set the soldering iron tip temperature
at 260°C (500°F) and increase the temperature as needed to obtain the desired result.
CONTROLLING HEAT
Controlling soldering iron tip temperature is not the key element in soldering. The key element
is controlling the heat cycle of the work. How fast the work gets hot, how hot it gets, and how
long it stays hot is the element to control for reliable solder connections.
THERMAL MASS
The first factor that needs to be considered when soldering is the relative thermal mass of the
joint to be soldered. This mass may vary over a wide range.
Each joint, has its own particular thermal mass, and how this combined mass compares with
the mass of the iron tip determines the time and temperature rise of the work.
SURFACE CONDITION
A second factor of importance when soldering is the surface condition. If there are any oxides
or other contaminants covering the pads or leads, there will be a barrier to the flow of heat.
Even though the iron tip is the right size and temperature, it may not be able to supply enough
heat to the joint to melt the solder.
THERMAL LINKAGE
A third factor to consider is thermal linkage. This is the area of contact between the iron tip
and the work.
Figure 2 shows a view of a soldering iron tip soldering a component lead. Heat is transferred
through the small contact area between the soldering iron tip and pad. The thermal linkage
area is small.
Figure 3 also shows a view of a soldering iron tip soldering a component lead. In this case,
the contact area is greatly increased by having a small amount of solder at the point of
contact. The tip is also in contact with both the pad and component further improving the
thermal linkage. This solder bridge provides thermal linkage and assures the rapid transfer of
heat into the work.
APPLYING SOLDER
In general, the soldering iron tip should be applied to the maximum mass point of the joint.
This will permit the rapid thermal elevation of the parts to be soldered. Molten solder always
flows from the cooler area toward the hotter one.
Before solder is applied; the surface temperature of the parts being soldered must be
elevated above the solder melting point. Never melt the solder against the iron tip and allow it
to flow onto a surface cooler than the solder melting temperature. Solder applied to a cleaned,
fluxed and properly heated surface will melt and flow without direct contact with the heat
source and provide a smooth, even surface, filleting out to a thin edge. Improper soldering will
exhibit a built-up, irregular appearance and poor filleting. For good solder joint strength, parts
being soldered must be held in place until the solder solidifies.
If possible apply the solder to the upper portion of the joint so that the work surfaces and not
the iron will melt the solder, and so that gravity will aid the solder flow. Selecting cored solder
of the proper diameter will aid in controlling the amount of solder being applied to the joint.
Use a small gauge for a small joint, and a large gauge for a large joint.
POST SOLDER CLEANING
When cleaning is required, flux residue should be removed as soon as possible, but no later
than one hour after soldering. Some fluxes may require more immediate action to facilitate
adequate removal. Mechanical means such as agitation, spraying, brushing, and other
methods of applications may be used in conjunction with the cleaning solution.
The cleaning solvents, solutions and methods used should not have affected the parts,
connections, and materials being cleaned. After cleaning, boards should be adequately dried.
RESOLDERING
Care should be taken to avoid the need for resoldering. When resoldering is required, quality
standards for the resoldered connection should be the same as for the original connection.
A cold or disturbed solder joint will usually require only reheating and reflowing of the solder
with the addition of suitable flux. If reheating does not correct the condition, the solder should
be removed and the joint resoldered.
WORKMANSHIP
Solder joints should have a smooth appearance. A satin luster is permissible. The joints
should be free from scratches, sharp edges, grittiness, looseness, blistering, or other
evidence of poor workmanship. Probe marks from test pins are acceptable providing that they
do not affect the integrity of the solder joint.
An acceptable solder connection should indicate evidence of wetting and adherence when the
solder blends to the soldered surface. The solder should form a small contact angle; this
indicates the presence of a metallurgical bond and metallic continuity from solder to surface.
(See Figure 4).
Smooth clean voids or unevenness on the surface of the solder fillet or coating are
acceptable. A smooth transition from pad to component lead should be evident.
Surface Mount Soldering
OUTLINE
This procedure covers the general guidelines for
soldering surface mount chip components. The following
surface mount chip components are covered by this
procedure. While all of these components are different,
the techniques for soldering are relatively similar
Chip Resistors
The component body of chip resistors is made out of
alumna; an extremely hard, white colored material. The
resistive material is normally located on the top. Chip
resistors are usually mounted with the resistive element
facing upwards to help dissipate heat.
Surface Mount Chip Component
Ceramic Capacitors
These components are constructed from several layers
of ceramic with internal metallized layers. Because metal
heats up much faster than ceramic, ceramic capacitors
need to be heated slowly to avoid internal separations
between the ceramic and the metal layers. Internal
damage will not generally be visible, since any cracks
will be inside the ceramic body of the component.
NOTE
Avoid rapid heating of ceramic chip capacitors during
soldering operations.
Figure 1: Prefill one pad with
solder.
Plastic Body
Another style of chip component has a molded plastic
body that protects the internal circuitry. There are a
number of different types of components that share this
type of exterior package. The termination styles for
plastic chip component packages vary considerably.
MELF
MELF - Metal Electrode Face cylindrical components.
These may be capacitors, resistors, and diodes. It can
be hard to tell them apart - since there is no universal
coloring or component designators printed on the
component bodies.
Figure 2: Place the soldering iron
tip at the junction between the
prefilled pad and component
lead.
PROCEDURE
1. Add liquid flux to one pad.
2. Prefill one pad with solder. (See Figure
1).
3. Clean the area.
4. Add liquid flux to both pads.
5. Place the component in position and
hold it steady with a wooden stick or
tweezers so that the soldering iron won't
push the component out of alignment.
Figure 3: Solder the other
opposite side of the component.
6. Place the soldering iron tip at the
junction between the prefilled pad and
component lead. Flow the solder until
the component drops down and is
soldered in position. Apply additional
solder as needed. (See Figure 2).
7. Remove the tip. Wait a moment for the
solder to solidify before soldering the
other side of the component. (See
Figure 3).
8. Clean, if required and inspect.
Chip Capacitors generally have
solid color bodies.